The district expanded as the built up area grew and stretched some distance into rural land.

The MPD was originally defined in reference to civil parishes and in 1946 was altered to correspond to local government districts.

The MPD has been used for other purposes during its existence, such as the boundary for coal tax and as a 'Greater London' statistical unit.

The Metropolitan Police Act 1839 recognised that the "boundary is... very irregular" and made it lawful to add any place in the Central Criminal Court District and also "any part of any parish, township, precinct or place" not more than 15 miles (24 km) from Charing Cross.

The enlarged district encompassed the metropolitan area and some parts of Essex, Kent, Hertfordshire and Surrey.

The other dashed lines show the boundaries of Essex (containing Loughton), Kent, Surrey, Middlesex, and Hertfordshire (going clockwise).

The former area of the Municipal Borough of Romford and Hornchurch Urban District, which had not previously been covered by the MPD but were now in Greater London, were added.

As constables of both forces are empowered throughout England and Wales, mutual assistance is a routine matter.